1. Field of the Invention
The present invention relates to a multiple amplifier circuit having input and output stages which employ different source voltages.
2. Description of the Related Art
In general, amplifier circuits that amplify an input signal to drive a loudspeaker or headphone include an input-stage amplifier for amplifying an input signal of low amplitude and an output-stage amplifier for delivering a signal of high amplitude to drive a loudspeaker or the like. In such an amplifier circuit, the input and output stage amplifiers need to use different source or supply voltages.
FIG. 2 is a view showing the configuration of a conventional amplifier circuit.
This amplifier circuit includes an input stage amplifier 1 that operates on a supply voltage Vdd (e.g., 2.4V) and an output stage amplifier 2 that operates on a supply voltage Vamp (e.g., 3.6V). The negative power supply terminals of these input and output stage amplifiers 1 and 2 are connected to a common ground potential GND. 
In the amplifier 1, a reference voltage Vref (e.g., Vdd/2) is supplied to the non-inverting input terminal, while an input signal Vin with its DC (direct-current) level being set at the reference voltage Vref is supplied to the inverting input terminal via a resistor 3. There is provided a feedback resistor 4 connected between the output terminal of the amplifier 1 or a node N1 and the inverting input terminal.
On the other hand, in the amplifier 2, a reference voltage Vs (e.g., Vamp/2) is supplied to the non-inverting input terminal, while a signal from the node N1 is given to the inverting input terminal via a series circuit made up of a capacitor 5 and a resistor 6 for blocking DC components. There is also a feedback resistor 7 connected between the output terminal and the inverting input terminal of the amplifier 2.
In this amplifier circuit, since both the non-inverting and inverting input terminals of the amplifier 1 are set at a DC potential of Vdd/2, the node N1 is also at a DC level of Vdd/2. This allows the input signal Vin to be amplified in the amplifier 1 and delivered at a voltage (DC level) centered about Vdd/2.
On the other hand, in the amplifier 2, since the non-inverting input terminal is set at Vamp/2, the inverting input terminal is also at a DC level of Vamp/2. The node N1 and the inverting input terminal of the amplifier 2 have different DC levels; however, since the capacitor 5 is connected therebetween, no DC current flows through the resistor 6. Therefore, no DC current flows through the feedback resistor 7, so that the output terminal of the amplifier 2 is at a DC level of Vamp/2. This enables the amplifier 2 to deliver from the output terminal a distortion-free output signal Vout with its maximum amplitude of Vamp.
Reference in this regard can be had to Japanese Patent Publications Kokai No. H07-46055 and No. H09-326645.
However, the conventional amplifier circuit, which has the capacitor 5 connected between the stages to block DC components, blocks not only DC components but also low-frequency components. To amplify lower-frequency components with high fidelity as well, the capacity of the capacitor 5 and the value of the resistor 6 must be increased. However, as in the case of integrated circuits, an increase in capacity of the capacitor 5 would cause an increase in layout area, which is not practical. Worse yet, to increase the value of the resistor 6, the value of the resistor 7 also needs to be increased at the same time. This leads to not only an increase in layout area but also a decrease in current due to the increase in value of the resistor 7, thereby making the entire circuit susceptible to noise.